Typically, firearms with scopes must be held in a certain posture if the scope is to accurately zero in on the target. Where the scope is mounted over the top of the barrel it cannot give an accurate sighting unless the sighting axis thereof and the firearm shooting axis lie essentially in a common vertical plane, i.e., unless the rifle and scope are not canted. This requirement for achieving ultimate marksmanship is elucidated, for example, in the article entitled "SUPER SCOPES" on pages 46-49 of Precision SHOOTING, vol. 37-No. 2, June 1991.